Upon completion of this chapter, the learner should be able to:
Describe the function/purpose of each cranial nerve, including motor, sensory, and autonomic function.
Select appropriate tests/measures for each cranial nerve.
Describe the effect of any specific cranial nerve deficit.
Correlate cranial nerve deficits to specific anatomic areas of the brainstem.
The term cranial nerve (CN) is an anatomical term for the peripheral nerves directly emerging from the brain or brainstem that provide innervation to structures around the head, face, neck, and body organs. CNs are responsible for numerous essential functional abilities, particularly sensory and motor processes for structures of the head, face, and neck as well as parasympathetic control. CN functions include the life-sustaining aspects of cardiovascular and pulmonary function, significant autonomic control, and the special senses of smell, vision, taste, and hearing. Equilibrium sensibility, the ability to sense the position and movement of the head in space, is the function of a CN. The adjective bulbar refers anatomically to the brainstem excluding the midbrain, especially the medulla oblongata. Bulbar symptoms are abnormal expressions of CN function, especially aspects of motor paralysis (bulbar paralysis) of the face, mouth, tongue, pharynx, and esophagus related to medulla pathology with impaired facial expression, speech production, and swallowing often seen with bulbar Amyotrophic Lateral Sclerosis (ALS) or progressive supranuclear palsy. Examination of CNs is often completed as a specific subset of the neurological examination because of the common aspects of head and neck functions and basic life-sustaining functions. This chapter will describe the tests and measures used to detect normal versus abnormal function of the CNs.
Examination of CN function is important as part of the evaluation of any patient with injury to the cerebrum or brainstem, whether from vascular disease, trauma, or other causes. The CN examination can reveal significant or subtle deficits often overlooked in the patient who has sustained orthopedic trauma such as a lower extremity fracture. Because of the focus on medical management of the orthopedic trauma, other health-care workers have often not previously detected such deficits. Abnormal results on a CN screen in this population, even just subtle signs, could be the first indication of brain injury related to the trauma.
Categories of Cranial Nerve Function
In general, CN function is either motor, sensory, or autonomic. With few exceptions, most CNs are “mixed” with both motor and sensory fiber components. As explained in the following sections, some motor and sensory components are designated as “special” because they carry special senses or motor innervation of structures derived from the embryonic branchial arches, particularly for motor functions related to eating and breathing.
Motor Cranial Fiber Categories
General somatic efferent (GSE) describes somatic motor innervation that results in voluntary movement. GSE includes movement of the eyes through CNs III, IV, and VI, and ...